1 On the Effectiveness of Peer-Assisted Internet TV Broadcasting Rossano Gaeta, Michele Garetto, Emilio Leonardi, Matteo Sereno Abstract—Despite the increasing popularity of peer-assisted video streaming applications, there have been few attempts to quantify the long-term impact of such bandwidth demanding applications on the core Internet infrastructure, as the audience grows to very large numbers (e.g. millions) of users. In this paper we analyze asymptotic scale effects of massively-deployed IPTV applications, addressing the concerns raised by many Internet Service Providers and backbone operators about the feasibility of large-scale video broadcasting over the Internet. In particular, focusing on the total network bandwidth consumption, we derive useful bounds valid for general topologies and accurate scaling laws for special cases of regular and random graphs, comparing the effectiveness of different architectural solutions. Using a snapshot of the Internet topology, we also investigate traffic concentration issues by evaluating the stress distribution on individual network elements. Finally, we analyze the transient behavior of distributed, application-layer algorithms to optimize the construction of multicast delivery trees. Our results show that the peer-to-peer approach is viable as long as a careful design of the overlay topology is performed by the application. I. I NTRODUCTION Today a new class of systems providing high-quality real- time video streaming is fast emerging and gaining more and more popularity. Thus, the Internet has to face a new challenge: delivering these applications to large set of users. Often termed as IPTV (Internet Protocol TeleVision), this scenario is very similar to video broadcasting, in which a very large number of users (up to millions) watches “TV” over the Internet. IPTV has initially been supplied by broadband operators exploiting IP multicast functionalities in small-scale environments through proprietary solutions, de-facto limiting the distribution of contents to each Internet Service Provider (ISP) network only. These IPTV systems follow the traditional client-server model of Internet protocols and services. How- ever, the limited deployment of IP multicast in the Internet, due to several concerns about its scalability, lack of authenti- cation and security mechanisms, and difficult integration with hierarchical routing, makes this solution unlikely to provide the infrastructure for future massively deployed IPTV. On the contrary, peer-assisted architectural solutions, in which users send data to each other using application-level multicast on overlay topologies, has emerged as a popular and promising approach to large-scale multimedia delivery, as demonstrated by an increasing number of successful applica- tions [1]. In this paper we will refer to peer-assisted streaming systems with the general term P2P-TV. Such applications are expected to change the way in which we watch TV, providing ubiquitous access to a vast number of channels, personalizing TV experience, and enabling roaming TV services. Further- more, the adoption of the P2P paradigm widely simplifies the operation and maintenance of the Internet infrastructure, * R. Gaeta, M. Garetto and M. Sereno are with Dipartimento di Informatica, Universit` a di Torino, Italy; E. Leonardi is with Dipartimento di Elettronica, Politecnico di Torino, Italy. pushing complexity to the end users, while at the same time relieving the bandwidth cost burden at the server. Although, from the users’ as well as from the content publishers’ point of view, this class of P2P applications possess very attractive and interesting properties, from the network operators’ point of view serious concerns exist about the ability of the Internet to support large scale P2P-TV systems, mainly because of the high bandwidth requirements and the lack of network-aware topology maintenance procedures in current applications. These concerns have recently emerged in news reports. For example, as reported in [2], some of the largest broadband providers in the UK are threatening to ”pull the plug” from the new iPlayer, which allows users to watch BBC programmes, because this service will place an intolerable strain on their networks. To improve the performance of P2P-TV systems, while at the same time reducing the impact of P2P traffic on the un- derlying physical infrastructure, several techniques have been proposed in the literature [3]–[6] that exploit peers proximity while building the overlay topology. However, application developers are only marginally interested to implement such techniques, especially due to the extra complexity required to integrate them in their systems. Moreover, it is still controversial whether network-aware techniques are indeed necessary to make P2P-TV scale to large numbers of users. Indeed, the majority of performance studies appeared so far in the literature rely on the assumption that the system bottleneck is due to the limited bandwidth on the users’ access lines (especially the upload bandwidth); hence, the main concern in the analysis and design of streaming systems has been to maximize the exploitation of peers access bandwidth. This is confirmed by recent measurement studies, which have revealed that popular applications such as PPLive and Soapcast, whose internal details are not public, seem essentially to ignore peer proximity [7]. This paper shifts the attention on the impact that large- scale P2P-TV applications may have on the underlying Internet infrastructure. Such impact is evaluated asymptotically, in the large users limit, showing how the network resources required to sustain P2P-TV traffic scale with the number of peers joining the streaming application, while considering at the same time the growth of the physical network infrastructure. Our findings show that a careful design of the application- layer distribution topology, taking into account network-layer metrics, can drastically reduce the impact of P2P-TV traffic on the Internet infrastructure. Actually, the gain of network-aware solutions increases almost linearly as the population of TV watchers increases. However, na¨ ıve approaches to optimize the formation of the overlay topology can incur excessive delays in very large systems, making it crucial to devise intelligent, cooperative schemes which have not yet been incorporated in current systems. II. NETWORK SCENARIO We represent the Internet topology at the router level adopt- ing the simplified model illustrated in Figure 1. We isolate last-tier ISPs (also referred to as access ISPs) from the core